When treating cellulose-containing pulp in a line, the pulp is subjected to chemical treatment, such as bleaching, whereupon the pulp is washed and subsequently dewatered. Afterwards, the dewatered pulp is diluted to enable onward transport by pumping to next treatment step in the line. The next treatment step can, for example, be further washing of the pulp with a new filtrate.
For washing and dewatering the pulp, a dewatering unit is used, which can consist of a conventional roll press comprising two press rolls, as disclosed in SE 505 539, e.g., or a conventional washing apparatus having a single dewatering drum, as disclosed in U.S. Pat. No. 3,014,589, e.g. The pulp dewatered in the dewatering unit is fed to a shredder screw, included in the dewatering unit, to be shredded there because of its high degree of dewatering, whereupon the shredded dewatered pulp is discharged via an outlet for onward transport and treatment. To enable onward transport by pumping to the next treatment step in a line, the dewatered pulp is diluted after its discharge from the outlet of the dewatering unit. Since the dewatered pulps has a high pulp consistency and is compact, the dilution has conventionally been effected by feeding the pulp via a dilution screw where dilution fluid is added at the same time as the pulp is shredded by the dilution screw, whereby the pulp obtains a suitable consistency for pumping. From the dilution screw the pulp is subsequently fed to a vertically extending stand pipe positioned vertically below the dilution screw and the outlet of the dewatering unit, where the pulp falls freely to a pump provided in the bottom of the stand pipe, by means of which the diluted pulp is transported to the next treatment step in the line.
Instead of a dilution screw for diluting the dewatered compact pulp from the dewatering unit, SE 526 292 discloses the use of a modified stand pipe, in which the pulp, when falling freely, is diluted by adding dilution fluid under pressure in the form of directed fluid jets. The stand pipe is positioned vertically below the outlet of the discharge unit and its shredder screw, whereby the pulp falls from the outlet to the stand pipe via a receiving pipe.
However, there are problems with the above-mentioned dilutions of pulp. The use of a dilution screw results in increased power requirements and increased investment costs, increased maintenance work and additional mechanical treatment of the pulp, which negatively affects the properties of the pulp. The use of the modified stand pipe of SE 526 292 can cause problems in the form of foaming or frothing, which is a known problem connected with negative effects, when the dilution fluid is added in the form a fluid jets, for instance when the pulp falls freely in a stand pipe.
Further, there are problems associated with the feeding of the pulp by means of said stand pipe. Pulp falling freely brings along a large amount of air which causes problems in the pump and in subsequent equipment. Upon blockage or jamming in the stand pipe, or upon breakdown of the pump provided in the bottom of the stand pipe, which results in that the stand pipe is filled up with pulp so that the pulp level rapidly rises in the stand pipe and overflow occurs in the stand pipe, the pulp level may rise all the way to the dilution screw, or to the shredder screw via the outlet of the dewatering unit, where the latter applies, e.g., to the solution of SE 526 292. The result is clogging in the provided screw, and as a result of this, the screw can be overloaded and breakdown can occur.